A wide variety of an insulating substrate is used for fabrication of a thick film circuit such as, for example, a hybrid integrated circuit, and various materials are available for formation of the insulating substrate. A typical example of such a material is aluminum oxide containing about 4% of miscellaneous substances by weight, and the miscellaneous substances are of silicon oxide, magnesium oxide and calcium oxide. The aluminum oxide substrate is superior in dielectric properties, thermal properties and the mechanical properties to the resin substrate, and, for this reason, the aluminum oxide substrate is shaped into various configuration and widely used by the circuit composers.
However, a problem is encountered in the prior-art substrate of aluminum oxide in heat radiation capability. This is because of the fact that aluminum oxide is not so large in thermal conductivity. Moreover, the thick film circuit is increased in integration density for achieving more complicated circuit behavior. The larger integration density the circuit has, the more heat it generates. Then, the problem becomes more and more serious.